PTFE has various excellent properties, such as heat resistance, light resistance, chemical resistance, electrical insulation, and tribological property, and is used in various fields mainly of mechanical, chemical, and electrical fields. PTFE fibers are one of articles (PTFE articles) containing PTFE and expected to be applied to various fields based on the various kinds of properties that PTFE has.
As the method for producing fibers, a melt spinning process and a wet spinning process are common. However, since PTFE has an extremely high melt viscosity of approximately 1010 to 1011 Pa·(1011 P to 1012 P) at 380° C., the melt spinning process cannot be used to produce the PTFE fibers. Moreover, since PTFE does not dissolve in most solvents except for specific solvents, it also is difficult to use the simple wet spinning process in which a PTFE solution obtained by dissolving PTFE in an appropriate solvent is extruded into a bath of a poor solvent and solidified.
Conventionally, an emulsion spinning process and a slit yarn process are known as the method for producing the PTFE fibers.
In the emulsion spinning process, there are a direct emulsion spinning process and a matrix spinning process. The matrix spinning process (as disclosed in JP 10 (1998)-273818 A, for example) mainly is used because it is more productive than the direct spinning process that requires extruding an aqueous emulsion of PTFE into a hydrochloric acid bath or a hydrogen chloride atmosphere. In the matrix spinning process, a material solution for spinning is prepared by adding a matrix material, such as viscose and cellulose, to a dispersion of PTFE particles, and then the material solution is extruded into a coagulation bath to be wet-spun. Thereafter, the fiber formed by the spinning is heat-treated (sintered) at a temperature equal to or higher than a melting point of PTFE so that the matrix material in the fiber is fired and removed as well as the PTFE particles dispersed in the matrix material being melted and fused to each other. Thus, the PTFE fiber can be formed. Usually, however, decomposing materials (carbides) of the matrix material remain in the PTFE fiber produced by this method, which may affect the physical and chemical properties that PTFE has intrinsically. For example, the PTFE fiber formed by the matrix spinning process has a color tone of brown to dark brown, which limits applications of the fiber. Moreover, since the matrix material and the decomposing materials thereof basically are unnecessary components for the PTFE fiber, a method for producing the PTFE fiber that requires no matrix material is desired.
The slit yarn process (as disclosed in U.S. Pat. No. 6,133,165 and U.S. Pat. No. 7,108,912, for example) can form the PTFE fiber by the following processes. (1) extrusion-molding a PTFE paste obtained by adding a forming aid to a PTFE fine powder into a sheet-like product, (2) removing the forming aid from the product, and then stretching the product to make a porous PTFE membrane, (3) processing mechanically the obtained porous membrane into a strip shape or a tape shape, and (4) stretching further the processed porous membrane. The reason why the paste is once extrusion-molded into a sheet shape or a film shape is because it is difficult to mold the paste directly into a fiber shape due to its high viscosity. With the slit yarn process, however, it is difficult to produce a fiber that is uniform in diameter and a long fiber (filament), although this depends on the way of the mechanical processing. It also is difficult to produce continuously a fiber from the raw material fine powder. Thus, the slit yarn process cannot be regarded as a highly productive production method.
As another method for producing the PTFE fiber, JP 2003-20515 A, for example, discloses a method in which an aqueous suspension of PTFE fine particles is pressurized to around 5 to 10 kgf/cm2 and sprayed from a capillary-shape die with an inner diameter of 200 μm to 400 μm so as to form the PTFE fine particles into a fiber, and then the fiber is dried and sintered. However, this method is unlikely to allow the production of the PTFE fiber that has excellent mechanical properties, such as strength and elastic modulus. Moreover, since the diameter of the capillary from which the suspension is sprayed probably is limited to the range of 200 μm to 400 μm for ensuring the pressure to be applied to the suspension, the degree of freedom in diameter is so low that the diameter of the producible PTFE fiber is 20 μm or less.
WO 2006/120967 discloses a method for obtaining an aggregate of PTFE particles containing water and a surfactant by applying a force that makes the particles approach or contact with each other to a dispersion of PTFE particles. WO 2006/120967 describes that by drying and/or sintering this aggregate, a string-shape PTFE product can be obtained, for example.
As described above, the conventional methods for producing the PTFE fiber require a component, such as the matrix material, that basically is unnecessary for the PTFE fiber, limit the producible fibers to short (staple) fibers, and limit the enhancement in productivity. Also, the PTFE fibers obtained by the conventional methods have poor mechanical properties and a low degree of freedom in diameter.